1. Field of the Invention
This invention relates to a charging member and a charging device for charging a member to be charged, an image forming apparatus, such as an electrophotographic apparatus or the like, having the charging device, and a process unit detachable relative to the apparatus.
2. Description of the Related Art
A corona discharger, such as a corotron, a scorotron of the like, having a wire electrode and a shield electrode surrounding the wire electrode and having an excellent charging uniformness has been widely used as means for uniformly charging the surface of an image carrying member, such as a photosensitive member, a dielectric member or the like, serving as a member to be charged in an image forming apparatus, such as an electrophotographic copier, an electrophotographic printer, a recording apparatus or the like.
However, the corona discharger has the following problems: An expensive high-voltage power supply is needed. Space is needed for the charger itself, as shield space for the high-voltage power supply, and the like. A large amount of corona products, such as ozone and the like, are produced, and hence additional means and mechanisms are needed in order to deal with the corona products. These factors result in a large and expensive apparatus.
In consideration of the above-described problems, the adoption of a contact charging method has recently been studied as an alternative to the corona discharger.
In contact charging, by contacting a contact charging member, to which a voltage (for example, a DC voltage of about 1-2 kV (kilovolts), or a superposed voltage composed of a DC voltage and an AC voltage) is applied from a power supply, to the surface of an image carrying member, serving as a member to be charged, the surface of the image carrying member is charged at a predetermined potential. Various contact charging methods have been devised, for example, a roller charging method (Japanese Patent Application Public Disclosure (Kokai) No. 56-91253 (1981)), a blade charging method (Japanese Patent Application Public Disclosure (Kokai) Nos. 56-104349 (1981) and 60-147756 (1985)), and a charging-and-cleaning method (Japanese Patent Application Public Disclosure (Kokai) No. 56-165166 (1981)), (U.S. Pat. No. 4,387,980 corresponds to Japanese Patent Document Nos. 56-91253 and 56-104349.)
Among such contact charging methods, the blade charging method is particularly effective for a small image forming apparatus because it provide for an inexpensive and compact apparatus.
One of the problems associated with the contact charging methods including the blade charging method is as follows: If a pinhole portion (i.e., a surface defect portion in a member to be charged) is present in an image carrying member, such as a photosensitive member or the like, a spark discharge is apt to occur between a contact charging member, to which a voltage is applied, in contact with the surface of the image carrying member in order to charge the surface of the image carrying member and the pinhole portion in the image carrying member. If such discharge occurs once, a so-called "charge leak" phenomenon will easily subsequently occur on the surface of the image carrying member wherein charged electric charges are held not only on the pinhole portion but also over the entire surface (the direction of the generatrix of a rotating image carrying member) of the charged region including the pinhole portion in contact with the contact charging member.
Fur purposes of background information, FIGS. 7(A) and 7(B) illustrates a model for explaining the charge leak phenomenon. In FIG. 7(A), a photosensitive member 1 serves as an image carrying member (a member to be charged) whose surface moves in the direction of the arrow. Pinhole portions P are present in the photosensitive member 1. A blade member 2 (hereinafter termed a "charging blade") of a contact charging member to which a voltage is applied is in contact with the surface of the photosensitive member 1 in order to charge the surface, FIG. 7(B) is an equivalent circuit of FIG. 7(A).
The pinhole portions P in the photosensitive member 1 have lower resistance values than other portions. Hence, when the charging blade 2 contacts the pinhole portions P or the surface of the charging blade 2 comes close to the pinhole portions P, spark discharges 5 are apt to occur between the charging blade 2 and the pinhole portions P. When the discharges S occur, potentials V.sub.A, V.sub.B, - - - V.sub.Z at respective portions applied on the surface of the photosensitive member 1 in the direction of the longitudinal direction of the photosensitive member 1 (the direction of the generatrix of the photosensitive member 1) become almost 0 V (volt). As a result, electric charge cannot be held on the surface of the photosensitive member 1 over the entire surface of the contact charging region including the pinhole portions P in contact with charging blade 2.
When the above-described charge leak portions are produced in the charging processing of the surface of the photosensitive member 1, image portions corresponding to the charge leak portions in an output image appear as white stripes in normal development and black stripes in reversal development, causing deterioration in image quality.
The pinholes P are apt to be produced, for example, during the production of an image carrying member (a member to be charged), such as a photosensitive member or the like, due to scratching, or due to dielectric breakdown. It is rather difficult to completely eliminate pinholes.
In order to prevent the above-described charge leaks, it is necessary to increase the electric resistance of the charging blade material. Since the charging blade 2 is pressed with a proper pressure utilizing rubber elasticity, the distance (the free length of the blade) between the distal end of a supporting member for the charging blade and a portion of the charging blade 2 in contact with the member to be charged must be considerably larger than the thickness of the blade 2. Accordingly, when a voltage is applied from the blade supporting member to the blade 2, the voltage drop in the blade 2 becomes large, causing a decrease in the potential of the portion of the blade 2 in contact with the member to be charged. Hence, it is necessary to attach a back electrode to the charging blade 2, but there has been no excellent means for producing a charging blade having a back electrode.
In order to prevent the charge leak, a resistive layer having a resistance value so large as not to produce charge leaks may be formed on the surface of the charging blade in contact with the member to be charged, but it is difficult to form the layer very accurately at a predetermined position.